It is becoming increasingly important for healthcare providers to determine accurately pulmonary functions and mechanics in patients due to the prevalence of pulmonary diseases such as chronic bronchitis and emphysema. Many of the tests for pulmonary functions and mechanics use the techniques of gas dilution. In these tests, the patient inspires a gas mixture of known composition, typically stored in pressurized gas tanks or cylinders supplied by gas manufacturers. Inside the patient's lungs, some of these gas components become diluted by the gas within the lungs prior to the inhalation and/or by diffusion of the gas components through the alveoli. Pulmonary function and lung mechanics information can be derived by measuring and analyzing the composition and volume of the gas the patient exhales. Trace gases in the inspired gas mixtures include carbon monoxide and acetylene (each of which is used to measure gas diffusion across the alveoli) and helium and methane (each of which is used to measure the dead space in the lung cavity and/or pulmonary testing device). As will be appreciated, carbon monoxide and acetylene absorb readily and rapidly into the bloodstream while helium and methane do not. In this case, the volumes of the carbon monoxide or acetylene component and the helium or methane in the inspired and/or expired gas are determined and used along with the known composition of the sample gas, to calculate the volume of carbon monoxide or acetylene absorbed by the lungs. Carbon dioxide concentration in the expired gas can also be measured to ascertain lung diffusion because the concentration of carbon dioxide is directly related to the amount of oxygen absorbed into the bloodstream.
A typical pulmonary testing device (e.g. Eagle™ from Ferraris Respiratory, Inc.) is shown in FIG. 1. The device 100 includes a breathing conduit 104 that includes a patient mouthpiece 108, first and second outlets 112 and 116 for the discharge of exhaled air and intake of ambient inhaled air, respectively, and a test gas intake assembly 120. Balloon valves 124 and 128 open and close respectively the outlets 112 and 116. The test gas intake assembly 120 comprises a diaphragm 132 biased by a spring 136 and connected to a closure arm 140 that opens and closes the test gas introduction port 144 of conduit 148 upon demand (referred to as a demand valve). When the patient closes the balloons 124 and 128 and inhales, the diaphragm 132 is drawn downwards and the closure arm 140 repositioned as shown by the dotted lines. In this position, the port 144 is opened, thereby introducing pressurized test gas of known composition into the device 100 via conduit 148. The test gas is subsequently inhaled by the patient via the patient mouthpiece 108.
The patient can exhale immediately or after a determined time, depending on the type of test being conducted. A series of gas component sensors denoted by block 152 measure the concentrations of various selected gas components in the inspired and/or expired gas stream(s). Additionally, a gas flow measuring device 156 measures the flow rate of the inspired and/or expired gas stream, as desired.
The volume of a gas component actually inspired by the patient is given by the following equation:VX=(VF×FX)−[(FX−FA)×VDS]where VF is the total gas volume actually inspired by the patient, FX is the fraction of the selected gas component in the tank volume, FA is the concentration of the selected gas component in the ambient atmosphere (or in the device 100 before the test), and VDS is the interior volume of device 100 (dead space volume).
If the gas component has negligible diffusion rate through the alveoli into the blood stream, exhaled gas concentration measurements will allow estimations of the lung volume at the start of inhalation. Using gas such as carbon monoxide that has a high diffusivity through the alveoli, exhaled gas concentration measurements will provide an estimate of the lung diffusion properties.
This device 100 can have disadvantages. For example, it can be complex, expensive, physically large and unwieldy, and difficult to use. It typically may not be used for a number of pulmonary tests, such as pulmonary tests conducted while the patient is exercising.
The pre-mixed gases used in pulmonary function and lung mechanics testing can also be costly. The logistics associated with the ordering, storing, and disposal of the specialized gas cylinders also add to the complexity of the operation of a pulmonary function laboratory.